Welding is a manufacturing technique to joint metal or other thermal plastic materials such as plastic by a heating method. After the material are welded, a welding seam is formed at the jointed section between two objects, in which the welding seam's depth, width and height, the weld throat's depth and the height difference between both welded sides are critical factors in relation to the strength of the welded structure. Taking a gas cylinder that contains high pressure gas as an example, a gas cylinder usually has a shroud and a foot ring welded attached to the cylinder body, and if they are poorly welded, the shroud or the foot ring of the gas cylinder may be cracked or detached to thus cause a disaster. Therefore the authorities have established particular strict standards with respect to the welding seam of the gas cylinders.
A welding seam is measured by a welding seam gauge which measures the welding seam's depth, width and height, the weld throat's depth or the height difference between both welded sides in accordance with the measuring scale on the gauge. However, the smallest unit of the measuring scale on the gauge is in a unit of 1 mm, so that the precision of the gauge is limited to 1 mm. Therefore it may become insufficient in a case when measuring smaller unit scales such as a 0.1 mm is required. In addition, since the scale numbers on the gauge are usually very small, it is difficult for a user to read the values during the measurement especially when a user still needs to record the measured values separately after each measurement. It thus takes time for the measurement.